Electroless metal deposition solutions comprise metal ions and a reducing agent for the metal ions. The reducing agent oxidizes on a catalytic surface, and provides electrons to the surface. These electrons, in turn, reduce the metal ions to the metal on the surface. This process may be written in the form of a chemical equation: EQU Red+Me.sup.n+ =Ox+Me.sup.o.
The term Red means the reducing agent, Me.sup.n+ refers to the metal ion, Ox means the oxidized form of the reducing agent and Me.sup.o refers to the reduced metal. This equation can be split into equations describing the two half reactions: EQU Red=Ox+ne.sup.-, and EQU Me.sup.n+ +ne.sup.- =Me.sup.o
where n is the valence of the metal ion and e.sup.- designates an electron.
In many electroless copper deposition solutions the reducing agent, Red, is an alkaline formaldehyde, an aqueous formaldehyde solution with a pH between 10 and 14. In the case of alkaline formaldehyde, Ox would be formate ion. Me.sup.n+ refers to the metal ion, e.g., a copper (II) ion, and Me.sup.o refers to the metal, copper. These general equations may be rewritten more specifically for a system with copper ions and alkaline formaldehyde as: EQU 2HCHO+4OH.sup.- =2HCOO.sup.- +2H.sub.2 O+H.sub.2 +2e.sup.-, and EQU CuL.sup.n+2 +2e.sup.- =Cu.sup.o +L.sup.n.
L designates the ligand necessary to prevent precipitation of basic copper compounds in alkaline solution and n refers to the valence of the ligand ion.
The half reaction of formaldehyde with hydroxide to produce electrons does not take place homogeneously in the bulk solution. It is a heterogeneous reaction which takes place on catalytic conductive surfaces such as copper. This reaction is called an anodic reaction. The half reaction for copper ions from the copper-ligand complex to copper metal is known as the cathodic reaction.
At the thermodynamic equilibrium, the rate of the anodic reaction, in the forward direction, EQU Red=Ox+e.sup.-,
is equal and opposite the rate of the same reaction in the opposite direction, EQU Ox+e.sup.- =Red;
and the potential of the electrode surface is the equilibrium potential. When the potential of the electrode surface is shifted to a more positive potential either by imposing a potential on the electrode from an external power supply, or by a second reaction with a more positive equilibrium potential simultaneously taking place on the same electrode, the rate of the forward reaction is no longer in equilibrium with the rate of the reverse reaction. The rate of the forward reaction increases or decreases as a function of the shift of the potential away from the thermodynamic equilibrium potential.
In many electrochemical processes, the anodic and cathodic reactions take place on separate electrodes, the anode and the cathode. In electroless metal deposition, the anodic and cathodic reactions take place on the same surface, so that at any instant one point on the surface may be considered anodic and another point on the surface considered cathodic, and the rate of the anodic reaction may be assumed to be equal to the rate of the cathodic reaction, with the electrons produced in the anodic reaction consumed in the cathodic reaction. In electroless metal deposition, the cathodic reaction, EQU Me.sup.n+ +ne.sup.- =Me.sup.o,
taking place on the same electrode with the anodic reaction shifts the anodic reaction to more positive potential (and the cathodic reaction to a potential more negative than its thermodynamic equilibrium potential). The potential where both the forward anodic and cathodic reactions are proceeding without an external voltage supply is a mixed potential, E.sub.mp, and is the deposition potential for electroless deposition.
At the mixed potential, the rates of the anodic and cathodic reactions are equal to each other, and can be measured as the deposition rate of the metal as mg/cm.sup.2 /hr which by Faraday's Law can be expressed as mA/cm.sub.2.
Copper deposits on substrates produced by electroless deposition or electroless deposition reinforced by electroplating are an important part of many processes used for the manufacture of printed circuits. Additive or fully additive printed wiring boards are made with a process which uses 100% electrolessly formed copper.
A specification, Mil Spec. P-55110-D, describes tests which measure the performance of printed circuits when subjected to conditions and environments the printed circuits will be exposed to during manufacture and use. In order to provide reliable printed circuits, the criteria for printed circuits in military and some commercial applications are based on the ability to meet the requirements of this specification.
Heretofore, electroless copper deposits on FR-4 epoxy glass material using the fully-additive method of making printed circuits have not been able to pass the Mil. Spec. P-55110-D thermal stress test. When exposed to this test, the plated-through holes would fracture during the 10-second exposure to the molten solder, usually at the intersections of the hole wall with the surface, the corners of the holes. These fractures (corner cracks) would usually fill with solder providing good electrical conductivity through the hole, but the integrity of the copper deposits were suspect and not acceptable for many applications. Although it is desirable to pass the Military thermal stress test, this has proved to be a difficult test to continuously pass in a production environment when manufacturing printed circuits using the additive method (electrolessly plated copper deposits) or the subtractive method (electroplated copper deposits). On the other hand, this test has been found to reliably predict performance of circuit boards under stress conditions encountered during use.
Prior art electroless copper formulations have been empirically derived and based on specific addition agents and conditions which were difficult to control and operate on a consistent basis. Many of the addition agents are present in parts per million or per billion and difficult to analyze and control. Furthermore, trace contaminants have been difficult to detect and have had major detrimental effects on deposition quality. The resulting copper deposits although acceptable for some commercial applications, have not been of sufficient quality to have broad acceptance in the industry.
In addition to the normal by-products formed during operation, chemical contamination can enter the plating solution through chemical additions, water supplies, air or from the work placed in the electroless copper bath. Many of the inorganic contaminants, such as iron, cuprous ions, silver, gold, antimony, arsenic and many other metals and their compounds, as well as many organic contaminants, can cause deleterious results for both bath operation and the quality of the copper deposits, even when only present in parts per million concentration.
For electroless copper deposition, it has been reported by Morishita et al., U.S. Pat. No. 4,099,974, that the concentration of the anodic reactants, formaldehyde and hydroxide, above a threshold, have little effect on the copper plating rate. Therefore Morishita et al. use only anodic reactant concentrations above the threshold. Under such conditions copper ion concentration does effect the plating rate.
The same observation, that plating rate is largely independent of the concentration of the anodic reactants, but depends mainly on the copper concentration has been reported by many authors. Donahue, Wong and Balla, J. Electrochemical Soc., vol. 127, p2340 (1980) summarize the data from a number of sources, showing the copper concentration is the major factor in the rate equation. In other words in electroless copper deposition solutions known and used in the art, the rate of the cathodic reaction, CuL.sup.n+2 +2e.sup.- =Cu.sup.o +L.sup.n, controls the rate of both reactions at the mixed potential.
The ductility, tensile strength and elongation needed in electroless copper plating for additive printed circuits has been widely studied. There is no agreement among the experts in the field on the numerical values of these properties necessary for additive printed circuits. However it has been widely held that these numerical values should be maximized in order to achieve additive printed circuit boards resistant to fissure formation in the copper deposits during soldering. The only common agreement that has been achieved among the experts is that the ductility of the copper deposits improves with increasing temperature of the electroless plating solution, as reported by Grunwald, Rhodenizer and Slominski, Plating, vol. 58, p1004 (1970).